We have been investigating casposons, proposed to be novel mobile elements that are the precursor of bacterial and archaeal adaptive immune systems. We expressed and purified the proposed transposase from the deep sea vent archaeon Aciduliprofundum boonei, and demonstrated that this casposase is indeed an active DNA integrase: it can integrate both short oligonucleotides and an excised mini-casposon into target DNA (3). We have recently determined the X-ray structure of a casposase bound to its target sequence, and discovered that when compared to structurally characterized Cas1-Cas2 integrases, casposon end binding resembled that of spacer overhangs and target binding resembles that of CRISPR repeats. Our structural results form the basis understanding the evolutionary process that is proposed to have converted a transposase into a CRISPR-Cas integrase. 3. Hickman and Dyda (2015) The casposon-encoded Cas1 protein from Aciduliprofundum boonei is a DNA integrase that generates target site duplications. Nucleic Acids Res. 43, 10576-10587.